Photosynthesis

Overview and Chemical Foundations of Photosynthesis

• Photosynthesis is the biochemical process by which plants, certain bacteria, and specific protistans utilize solar radiant energy to synthesize glucose from carbon dioxide and water.

• The generated glucose serves as an essential energetic precursor that can be converted into pyruvate, releasing adenosine triphosphate ($ATP$) through cellular respiration.

• Molecular oxygen ($O_2$) is concomitantly produced and released as a byproduct.

• The overarching transformation is summarized by the word equation:

  • $\text{carbon dioxide} + \text{water} \xrightarrow[\text{chlorophyll}]{\text{sunlight}} \text{glucose} + \text{oxygen}$

• The conversion of incident sunlight into stable chemical bond energy is mediated by the specialized green pigment chlorophyll.

Chlorophyll and Photosynthetic Pigments

• Chlorophyll is a complex organic molecule that occurs in several modified molecular species across diverse photosynthetic taxa.

• Chlorophyll a is universally present in all photosynthetic organisms.

• Accessory pigments function to absorb electromagnetic wavelengths that chlorophyll a fails to capture, expanding the action spectrum of photosynthesis:

  • Chlorophyll b (as well as chlorophyll c, d, and e found in various algae and protistans).

  • Xanthophylls.

  • Carotenoids (such as beta-carotene).

• Absorption characteristics of chlorophyll a:

  • Strongly absorbs radiant energy in the violet-blue and reddish orange-red wavelengths.

  • Absorbs very little energy from intermediate wavelengths (the green-yellow-orange band), reflecting green light.

• Molecular architecture of chlorophyll:

  • A lipid-soluble hydrocarbon tail with the chemical composition $(C_{20}H_{39}-)$.

  • A planar hydrophilic head featuring a coordinated magnesium ion ($Mg^{2+}$) at its centre.

  • Different forms of chlorophyll vary based on specific chemical side-groups attached to the hydrophilic head.

  • The hydrocarbon tail and the hydrophilic head are covalently joined by an ester bond.

Leaf Structure and Physiological Gas Exchange

• Plants represent the sole group of photosynthetic organisms possessing leaves, although not all plant species have leaves.

• Morphologically and functionally, a leaf acts as a specialized solar collector packed with photosynthetic cells.

• Material exchange in leaf tissues:

  • Raw materials: Water ($H_2O$) and carbon dioxide ($CO_2$) enter the photosynthetic cells of the leaf.

  • Synthesized products: Carbohydrate sugars and molecular oxygen ($O_2$) exit the leaf.

• Vascular water supply:

  • Water is absorbed by root systems and transported upward to the foliage through specialized, reinforced plant cells known as xylem vessels.

• Adaptations against desiccation:

  • Terrestrial land plants must prevent excessive water loss while permitting gas exchange, having evolved microscopic pores called stomata (singular: stoma).

  • The outer epidermis of the leaf is coated with a protective, waxy, water-impermeable layer termed the cuticle.

  • Carbon dioxide cannot diffuse through the waxy cuticle; it gains entry into the interior leaf mesophyll exclusively through the stoma, which is flanked and regulated by two guard cells.

  • Molecular oxygen produced within the chloroplasts can only exit the leaf through opened stomata.

  • Transpirational penalty: The open stomatal pathway inevitably leads to substantial transpirational water loss. For example, Cottonwood trees lose approximately $100\,\text{gallons}$ (about $450\,dm^3$) of water per hour during hot desert days.

Chloroplast Ultrastructure and Compartmentalization

• The structural unit of photosynthesis is the thylakoid, consisting of a flattened, membrane-bound sac or vesicle containing embedded photosynthetic pigments and electron carriers.

• Both photosynthetic prokaryotes and photosynthetic eukaryotes possess thylakoids; however, only eukaryotes organize them within a membrane-delimited organelle called the chloroplast.

• Thylakoids are stacked like pancakes into multi-layered structures termed grana (singular: granum).

• The aqueous, enzyme-rich fluid filling the interior regions between grana is known as the stroma.

• Membrane systems and internal compartments:

  • While the mitochondrion possesses two distinct membrane systems, the chloroplast contains three distinct membrane systems: the outer membrane, the inner membrane, and the thylakoid membrane.

  • These three membranes define three physically separate internal compartments: the intermembrane space, the stroma, and the thylakoid compartment (lumen).

Photochemical Mechanisms and Excited States

• When light energy strikes a chlorophyll molecule, its electrons absorb photon energy and are promoted to higher electronic orbital levels, a state known as photoexcitation.

• When sufficient photon energy is absorbed, the electron is ejected from the molecule, leaving behind a positively charged chlorophyll radical ion; this event is defined as photoionisation.

• Behavior in isolated chlorophyll versus an intact chloroplast:

  • In an isolated chlorophyll molecule, an excited electron drops back down to its lower ground-state energy level, dissipating the absorbed energy as emitted fluorescence (light) and heat.

  • In an intact chloroplast, the excited electron is captured by an electron acceptor, preventing immediate decay to ground level. Electrons supplied from an electron donor replenish the positive hole left in the chlorophyll molecule.

• Organization of a photosystem core:

  • Inside chloroplast thylakoid membranes, each chlorophyll molecule is functionally coupled to an electron acceptor and an electron donor.

  • These three components collectively constitute the core of a photosystem.

  • Two electrons from a photoionised chlorophyll molecule are transferred to the primary electron acceptor.

  • The resulting positively charged chlorophyll ion extracts a pair of replacement electrons from an adjacent electron donor, such as water ($H_2O$).

• Primary chemical reaction categories in photosynthesis:

  • Condensation reactions: Reactions responsible for eliminating water molecules, notably including phosphorylation (the enzymatic addition of an inorganic phosphate group to an organic compound).

  • Oxidation/reduction (redox) reactions: Chemical reactions involving the directional transfer of electrons between intermediate electron donors and acceptors.

Overview of the Two Stages of Photosynthesis

• Photosynthesis operates through two distinct, interdependent stages: the light-dependent reactions and the light-independent reactions.

• The light-dependent reactions:

  • Site: Localized strictly within the grana / thylakoid membranes.

  • Requirement: Direct input of solar light energy.

  • Function: Synthesize chemical energy-carrier molecules ($ATP$ and reduced nicotinamide adenine dinucleotide phosphate) that fuel the subsequent stage.

  • Key events:

    • Trapping of light energy by chlorophyll to synthesize $ATP$ via photophosphorylation.

    • Simultaneous photolytic splitting of water into molecular oxygen, hydrogen ions (protons), and free electrons:

      • $2H_2O \rightarrow 4H^+ + O_2 + 4e^-$

    • Reaction of liberated electrons and protons with the electron carrier nicotinamide adenine dinucleotide phosphate ($NADP^+$), reducing it from its oxidized state ($NADP^+$) to its reduced state ($NADPH$):

      • $NADP^+ + 2e^- + 2H^+ \rightarrow NADPH + H^+$

• The light-independent reactions:

  • Site: Occur within the stroma of the chloroplast.

  • Requirement: Does not require light directly; consumes the $ATP$ and $NADPH$ produced by the light-dependent reactions.

  • Function: Reduces inorganic carbon dioxide ($CO_2$) into organic carbohydrates, initially producing the three-carbon sugar glyceraldehyde 3-phosphate.

Non-Cyclic Photophosphorylation and the Z Scheme

• Non-cyclic photophosphorylation generates both adenosine triphosphate ($ATP$) and reduced nicotinamide adenine dinucleotide phosphate ($NADPH$).

• Photosystem complexes:

  • Photosystem II (PSII), containing reaction centre P680.

  • Photosystem I (PSI), containing reaction centre P700.

  • Nomenclature: PSII acts prior to PSI in the physiological electron transport sequence; it is named "II" because it was discovered second chronologically.

• The Z scheme pathway:

  • When the progression of oxidation-reduction energy levels is mapped out graphically, the sequential electron transitions form a characteristic "Z" shape.

  • Sufficient Gibbs free energy is released during downhill electron transit along the thylakoid membrane transport chain to drive the synthesis of $ATP$ from $ADP$ and inorganic phosphate.

• Stepwise mechanism of non-cyclic photophosphorylation:

  • Step 1: Incident photons excite PSII (P680), inducing photoionisation; two excited electrons are transferred to a primary electron acceptor.

  • Step 2: Photolysis of water (which acts as the physiological electron donor) splits water molecules into oxygen, hydrogen ions ($H^+$), and free electrons ($e^-$):

    • $H_2O \rightarrow \frac{1}{2}O_2 + 2H^+ + 2e^-$

  • Step 3: The liberated electrons from water are transferred to the positively charged chlorophyll reaction centre of PSII, refilling its electron deficit.

  • Step 4: The primary electron acceptor passes the two electrons down a thylakoid electron transport chain; the terminal electron acceptor of this chain is Photosystem I (PSI).

  • Step 5: Additional photon absorption by PSI (P700) elevates these electrons to an even higher energy level, driving them through ferredoxin to reduce $NADP^+$ into $NADPH$.

Condensation Synthesis of ATP and Redox Coenzymes

• Phosphorylation mechanism:

  • The synthesis of $ATP$ from adenosine diphosphate ($ADP$) and orthophosphoric acid ($H_3PO_4$) is a condensation reaction.

  • A hydroxyl group from phosphoric acid and a hydrogen atom from $ADP$ undergo elimination as a water molecule ($H_2O$), producing an energy-rich phosphoanhydride bond.

• Coenzyme structures:

  • $NADP^+$ represents the oxidized dinucleotide carrier, possessing a positively charged quaternary nitrogen on its nicotinamide ring.

  • $NADPH$ represents the reduced form, carrying two additional electrons and one additional proton on the nicotinamide ring, neutralizing the positive charge and storing reducing equivalents for stroma-based carbon fixation.

Chemiosmosis and Proton Gradient Generation

• The multi-protein assemblies of non-cyclic photophosphorylation are integrated directly into the chloroplast thylakoid membrane.

• Electrochemical proton gradient formation:

  • Electrons moving through intermediate electron transport chain carriers provide the free energy required to actively pump hydrogen ions ($H^+$) from the stroma across the thylakoid membrane into the thylakoid interior compartment (lumen).

  • Photolysis of water simultaneously deposits $H^+$ directly into the thylakoid lumen.

  • The consumption of stromal $H^+$ during the reduction of $NADP^+$ to $NADPH$ further depletes stroma proton levels.

  • These actions generate an electrochemical proton gradient, characterized by a significantly higher concentration of $H^+$ within the thylakoid lumen relative to the stroma.

• ATP production via ATP synthase:

  • The thylakoid membrane is impermeable to free ions; $H^+$ can only diffuse down its steep electrochemical gradient through the catalytic channel of the enzyme ATP synthase.

  • This passive chemiosmotic diffusion of protons through ATP synthase powers the condensation phosphorylation of $ADP$ and inorganic phosphate ($P_i$) into $ATP$.

Cyclic Photophosphorylation

• Energetic necessity:

  • Non-cyclic photophosphorylation transfers electrons from water to $NADP^+$, yielding $ATP$ and $NADPH$.

  • However, the light-independent reactions require significantly more $ATP$ than $NADPH$ to synthesize hexose sugars and regenerate substrate molecules.

  • This quantitative deficit of $ATP$ is satisfied by cyclic photophosphorylation.

• Pathway of cyclic electron transport:

  • Involves only Photosystem I (P700); Photosystem II is excluded.

  • Photon absorption excites electrons in PSI, transferring them to its primary electron acceptor.

  • Instead of proceeding to NADP reductase, the excited electrons are diverted backward into the intermediate electron transport chain located between PSII and PSI.

  • Electrons cascade down through this electron transfer system, releasing free energy to pump protons across the membrane, and eventually return to the P700 reaction centre of PSI.

• Distinctions from non-cyclic photophosphorylation:

  • No photolysis of water occurs; consequently, no molecular oxygen ($O_2$) is evolved.

  • Electrons never reach $NADP^+$; thus, no $NADPH$ is generated.

  • The cycle functions solely to drive proton pumping across the thylakoid membrane, generating extra $ATP$ via chemiosmosis.

The Light-Independent Reactions (The Calvin Cycle)

• The light-independent process (traditionally called the Dark Reaction) takes place in the chloroplast stroma.

• Carbon fixation:

  • Inorganic carbon dioxide ($CO_2$) from the atmosphere (or dissolved in water for aquatic and marine organisms) is captured and incorporated into organic compounds through the addition of hydrogen.

  • Living cells cannot directly use photonic radiant energy to power cellular work; through the photosynthetic cascade, they convert it into stable $C-C$ covalent bond energy that can later be metabolized by glycolysis and cellular respiration.

• Step 1: Carboxylation of RuBP:

  • Carbon dioxide ($CO_2$, a 1-carbon molecule) combines with the 5-carbon pentose sugar ribulose 1,5-biphosphate (RuBP).

  • This reaction forms an unstable 6-carbon reaction intermediate.

  • The unstable 6-carbon molecule spontaneously breaks down into two molecules of glycerate 3-phosphate (GP), also referred to as 3-phosphoglyceric acid (3PG) or phosphoglycerate (PGA).

  • PGA/GP constitutes the first stable chemical product of the Calvin cycle.

• Step 2: Phosphorylation and Reduction:

  • Each glycerate 3-phosphate (GP) molecule is phosphorylated by $ATP$ to form glycerate diphosphate (1,3-bisphosphoglycerate).

  • Glycerate diphosphate is subsequently reduced by $NADPH$ into glyceraldehyde 3-phosphate (GALP), also known as phosphoglyceraldehyde (PGAL, a 3-carbon phosphorylated sugar).

• Stoichiometry of hexose synthesis and RuBP regeneration:

  • For each pair of GALP molecules produced:

    • One molecule represents the initial end product of photosynthesis, which is rapidly converted into glucose, other carbohydrates, lipids, or amino acids.

    • One molecule is recycled through a series of enzymatic steps to regenerate RuBP.

  • To achieve net glucose production over multiple cycles:

    • Twelve molecules of glyceraldehyde phosphate (PGAL / GALP, 3-C) are produced.

    • Two of these twelve PGAL molecules are removed from the cycle and condensed to synthesize one molecule of glucose ($C_6H_{12}O_6$, 6-C).

    • The remaining ten PGAL molecules ($10 \times 3\,\text{carbons} = 30\,\text{carbons}$) are converted using additional $ATP$ energy to regenerate six molecules of ribulose 1,5-biphosphate ($6 \times 5\,\text{carbons} = 30\,\text{carbons}$), allowing the Calvin cycle to restart.

Overall Integration of Photosynthetic Stages

• Integration of solar energy, water, and gas inputs:

  • Incident light penetrates the chloroplast, where chlorophyll located within grana captures the radiant energy.

  • A portion of this captured energy is used to split water into hydrogen and oxygen.

  • Molecular oxygen is released from the plant into the surrounding atmosphere.

  • The liberated hydrogen is bound to carrier molecules ($NADPH$) and transported into the stroma.

  • The remaining captured light energy is stored in $ATP$ molecules and transported into the stroma.

  • Carbon dioxide ($CO_2$) enters the leaf tissue via stomata and diffuses into the stroma.

  • Stromal enzymes utilize $ATP$ and $NADPH$ to combine hydrogen with carbon dioxide, synthesizing carbohydrates.

  • Synthesized carbohydrates are subsequently exported to the remaining cells of the plant for growth and metabolic activity.

Limiting Factors Controlling the Rate of Photosynthesis

• The rate of photosynthesis is regulated by environmental variables termed limiting factors, primarily light intensity, carbon dioxide concentration, and temperature.

• Light intensity and wavelength:

  • At low to moderate intensities, increasing light intensity produces a directly proportional increase in the rate of the light-dependent reactions and overall photosynthesis.

  • At high light intensities, the rate eventually reaches a plateau, where further increases do not increase the photosynthetic rate because another factor has become limiting.

  • Wavelength absorption efficiency:

    • Chlorophyll a is active within both photosystems.

    • Photosystem I (PSI) absorbs light energy most efficiently at a wavelength of $700\,nm$.

    • Photosystem II (PSII) absorbs light energy most efficiently at a wavelength of $680\,nm$.

    • Illumination concentrated in these specific absorption wavelengths produces the highest photosynthetic rates.

• Carbon dioxide concentration:

  • An increase in ambient carbon dioxide concentration accelerates the rate of carbon fixation during the light-independent reactions.

  • The photosynthetic rate increases progressively with rising $CO_2$ levels until the active sites of carboxylating enzymes become saturated or another factor becomes limiting.

• Temperature:

  • Photosynthesis relies heavily on enzyme-catalyzed biochemical reactions, particularly within the stroma.

  • As temperature increases toward the physiological optimum, the kinetic energy of reactants and enzymes increases, accelerating the overall photosynthetic rate.

  • When the temperature exceeds the optimum, thermal denaturation of the enzymes occurs, causing reaction rates to drop sharply until photosynthesis stops completely.